The present invention relates to solid state modulators and more particularly to pulse modulators for microwave radar transmitters Typically, for high powered systems, the transmitter is modulated by a high powered vacuum tube typically a tetrode switch tube. Such a tetrode switch tube suffers from the disadvantages of low efficiency, a long warm up period, and large standby power consumption. Typically for example, for each 1 KW average RF power output, approximately 1.2 KW average power is dissipated by each tetrode switch tube in a 0.008 duty cycle. Additionally, such tubes have the disadvantages of short lifetimes and the necessity for high voltage power supply in order of 17 KVDC requiring high voltage power supplies which are bulky, and require high voltage protective circuitry and specialized high voltage components.
Most crossed-field microwave tubes used in such radar transmitters require a high voltage constant current cathode pulse for proper operation. The tetrode switch tubes are often used for generating such a modulated cathode pulse by applying a large negative voltage pulse to the cathode of the microwave tube. The modulator most commonly used is the soft tube line type. However, line type modulators are limited in their ability to operate with large variations in pulse width. Where large variations in pulse width or spacing are required, a vacuum (hard) switch tube is usually used. For such cases the crossed-field amplifiers (CFA) are modulated by a large tetrode switch tube. The switch tube not only forms the cathode voltage pulse but provides constant current regulation for the top of the pulse. This constant current regulation compensates for variations in power supply voltage and for variations in the impedance of the CFA with RF frequency. To provide sufficient dynamic range for the constant current regulation, a sizeable voltage drop must be maintained across the switch tube and this results in significant power dissipation in the switch tubes which reduces the overall transmitter efficiency. For example, for each 1 KW of RF output power, 1.3 KW is dissipated in the CFA anode, 0.7 KW is dissipated in the switch tube anode and, 0.75 KW is used in the switch tube heater. Thus, more power is dissipated in the switch tube then is converted to RF. The overall efficiency of the transmitter can be greatly improved and the other disadvantages overcome if a more efficient switching system and a non-dissipative constant current regulator is used.
Solid state devices provide high efficiency switching and good reliability. Such devices also provide instant turn on and simplified circuitry. To date, many high power solid state modulators have used avalanche devices, such as SCR's, triacs, etc. However, avalanche devices are difficult and relatively slow to turn-off. Transistors having sufficient switching speed and power capability have not been available at low cost to make a high power all transistor modulator practical for the powers expressed herein. However, power MOSFET transistors capable of switching 50 amperes at 500 volts (25 kw) at 50 nanoseconds have recently become available at reasonable cost. For a CFA requiring 13 kv, 23 ampere (300 kw) pulse, 20 power FET's can provide sufficient pulse power to operate such a CFA since twenty transistors cost less than one switch tube. Thus, for high power CFA's on the order of 1.0-1.25 MW with higher voltage requirements of the order of 40 KV, such solid state modulators are lighter, cheaper and capable of switching larger amounts of power at high duty cycles with greater efficiency than would otherwise be practicable.